Acquired Immune Deficiency Syndrome ("AIDS"), a fatal human disease, is generally considered to be one of the more significant diseases to affect humankind, and has affected numerous individuals worldwide. The disease appears to have originated in Africa and then spread to other locations, such as Europe, Haiti and the United States. AIDS began to be recognized as a distinct new disease in about the mid-1970s. The number of reported cases in the U.S. alone exceeds 100,000. The number of persons in just the U.S. who are infected has been estimated to be greater than one million.
Due to the devastating effect of AIDS on patients and indications that the disease is spreading, much effort has been devoted to elucidate and identify the cause of the disease. Epidemiological data suggested that AIDS is caused by an infectious agent that is transmitted by exposure to blood or blood products. Groups reported to be at greatest risk of contacting AIDS include homosexual or bisexual males and intravenous drug users. Hemophiliacs who receive blood products pooled from donors and recipients of multiple blood transfusions are also at risk.
AIDS is a disease that damages the body's immune system, leaving victims susceptible to opportunistic infections, malignancies or other pathological conditions against which a normal immune system would have protected the subject. After patients develop symptoms of AIDS, death generally occurs within 2-3 years of diagnosis. Clinical manifestations of the disease in its final stage include a collapse of a patient's immune defenses (which generally involves a depletion of helper T cells) accompanied by the appearance of a Kaposi sarcoma and/or various opportunistic infections. The pronounced depression of cellular immunity that occurs in patients with AIDS and the quantitative modifications of subpopulations of their T lymphocytes suggests that T cells or a subset of T cells are a central target for the infectious agent.
The etiology of AIDS and related disorders has been identified as being associated with infection by a new class of lymphotrophic retrovirus termed human immunodeficiency virus (HIV; known previously as HTLV or LAV). It appears that the virus is spread when body fluids, such as semen, vaginal fluids or blood, from an infected individual are passed to an uninfected person. As noted above, AIDS is characterized by a disorder associated with an impaired cell-mediated immunity and lymphopenia, in particular, depletion of those T cells that express the T4 (CD4) glycoprotein. This is due to the fact that HIV preferentially infects the CD4 lymphocyte population (CD4 cells). Both the binding of virus to susceptible target cells and the cell fusion that is a characteristic manifestation of HIV-induced cytopathology involve specific interactions between glycoproteins in the viral envelope and the cell surface of CD4 cells.
HIV contains two heavily glycosylated external envelope proteins, gp120 and gp41, which mediate attachment of virions to glycosylated cell surface receptor molecules. These glycoproteins are encoded by the env gene and translated as a precursor, gp160, which is subsequently cleaved into gp120 and gp41. Gp 120 binds to the CD4 protein present on the surface of helper T lymphocytes, macrophages, and other cells, thus determining the tissue selectivity of viral infection.
The CD4 protein is a glycoprotein of approximately 60,000 molecular weight and is expressed on the cell membrane of mature, thymus-derived (T) lymphocytes, and to a lesser extent on cells of the monocyte/macrophage lineage. CD4 cells appear normally to function by providing an activating signal to B cells, by inducing T lymphocytes bearing the reciprocal CD8 marker to become cytotoxic/supressor cells, and/or by interacting with targets bearing major histocompatibility complex (MHC) class II molecules. The CD4 glycoprotein in addition to playing an important role in mediating cellular immunity also serves as the receptor for HIV. A variety of proposed therapeutic approaches have been based upon an attempt to disrupt the interaction of HIV gp120 with T cell CD4.
Once HIV has infected a cell, it replicates to increase the number of copies of the virus. Replication of the HIV genome proceeds by a series of enzymatic reactions involving two virus-encoded enzymes, reverse transcriptase ("HIV RT") and integrase, as well as host cell-encoded DNA polymerases and RNA polymerase. HIV RT polymerizes deoxyribonucleotides by using viral RNA as a template and also acts as a DNA polymerase by using the newly synthesized minus strand DNA as a template to produce a double-stranded DNA. More specifically, HIV RT copies the viral RNA to yield an RNA-DNA hybrid. The RNA strand of the hybrid is degraded and then the viral polymerase copies the resultant single-stranded DNA to yield a double-stranded DNA. The latter is integrated into the host cell genome.
Because of the essential role of HIV RT in the invasion of a host organism by the virus, therapeutic approaches have been based upon an attempt to inhibit HIV RT or to incorporate nucleoside analogs that terminate viral DNA synthesis. The most commonly used drugs against HIV RT are chain terminators. These molecules are presumably incorporated into the polynucleotide chain by HIV RT, but are unable to be extended on subsequent nucleotide additional steps. For example, azidothymidine ("AZT"), one of the most commonly used drugs for the treatment of AIDS, is directed against HIV RT. However, even these inhibitors of HIV RT have had limited success because of the extensive genetic variation and high mutation rate of HIV. Therefore, by rapid evolution of HIV, mutations in HIV RT arise frequently in infected individuals and render the virus resistant to HIV RT inhibitors. This is a significant drawback to conventional therapies.
Although a few drugs such as AZT have prolonged the lives of some individuals with AIDS, there is presently no cure for AIDS. Therapeutic agents are needed for all stages of AIDS infections, to block action of the virus once infection has occurred, and to restore full function in patients whose immune systems have been damaged. Due to the limited success for previously suggested compositions for the treatment of AIDS, there is a need in the art for new therapies and a method to screen for drugs which may be useful in the new therapies. The present invention fills this need, and further provides other related advantages.